For the contest, since it's a comic maybe a fan art contest? Or maybe you could also mix in short story contest too.
For prizes... I don't know about your budget but if you don't have it may be let them choose a character/s you would draw for them for free or something. If you have some money, maybe some postcard with your character drawing or print, but it's up to you. BTW, I see some peoples do some contest/giveaway with things unrelated to their comic or story, for examples figurine of some character, doll, etc.
Anyway, good luck with your contest, but think things thorough about your prizes before starting it. Best of luck. ^^